grpc service

[majst01]

In the long run we want to replace the ipam library usage in metal-api with a dedicated microservice. With this PR we provide a grpc service for this purpose.

replaces #46

[codecov-commenter]

Codecov Report

Merging #82 (5a76999) into master (9d82af9) will decrease coverage by 2.64%.
The diff coverage is 56.05%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   74.43%   71.79%   -2.65%     
==========================================
  Files           9       10       +1     
  Lines         935     1092     +157     
==========================================
+ Hits          696      784      +88     
- Misses        130      191      +61     
- Partials      109      117       +8     

Impacted Files	Coverage Δ
etcd.go	63.90% <0.00%> (-0.98%)	⬇️
memory.go	93.33% <0.00%> (-3.22%)	⬇️
mongodb.go	62.38% <0.00%> (-1.17%)	⬇️
redis.go	72.34% <0.00%> (-1.58%)	⬇️
sql.go	61.17% <0.00%> (-1.48%)	⬇️
pkg/service/ipam-service.go	59.86% <59.86%> (ø)

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[Gerrit91]

Looks fine to me, only minor comments added.